An electric power steering system includes an assisting electric motor and an electric power steering controlling portion for controlling the assisting electric motor. In a situation where the controlling portion turns to an abnormal state, the electric power steering controlling portion stops the controlling of the assisting electric motor without discomforting a driver while operating the steering wheel. For example, in a system described in JP2001-66200A (Patent document 1), in a situation where external noises are mixed with a torque sensor circuit signal and a level of output of a signal processing circuit abnormally rises or falls even for a short time, assist force is not abruptly reduced, but is gradually reduced.
The electric power steering system includes, in addition to the assisting electric motor and the electric power steering controlling portion described above, a power supply source for supplying power to the electric power steering controlling portion. Conventionally, in a situation where the power supply source turns to an abnormal state, the electric power steering system completely stops the controlling of the assisting electric motor and stops operating regardless of abnormal state conditions.
However, assisting the steering force by the electric power steering system is an important function in recent vehicles. Accordingly, it is desired that a state, where the electric power steering system is functioning, be retained as much as possible.
A need thus exists for an electric power steering system, in which an electric power steering controlling portion does not completely stops controlling of an assisting electric motor and keeps a state where the electric power steering controlling portion operates according to abnormal state conditions even when a power supply source turns to an abnormal state. The present invention has been made in view of the above circumstances and provides such an electric power steering system